The distribution and abundance of kelp forests flanking Santa Monica <br /><br />Bay are spatially and temporally variable and have shown an overall decline in <br /><br />biomass over the past few decades, especially along the northern arm of the bay <br /><br />in Malibu. Our objective was to quantify the within patch abundance of giant <br /><br />kelp Macrocystis pyrifera (L.) C. Ag. fringing northern Santa Monica Bay and <br /><br />to measure a suite of physical factors that may influence its local distribution and <br /><br />abundance. For comparison, these factors were also measured for the southern <br /><br />arm of the Bay along the Palos Verdes Peninsula, an area characterized by larger, <br /><br />more persistent kelp forests. Of the measured factors, the amount and stability of <br /><br />suitable substrate and the sediment type appear to have the largest influence on <br /><br />kelp density. The amount of hard substrate available for kelp recruitment and <br /><br />growth is higher along the Palos Verdes peninsula as compared to the Malibu <br /><br />coastline, but both regions' rocky reefs demonstrate temporal shifts in areal extent. Furthermore, sediment size also varies along the two coasts with Malibu sediment consisting of a lower percentage of larger particles which typically are resuspended less than smaller, more mobile particles. These abiotic factors may strongly regulate the size of Macrocystis pyrifera populations in the Santa Monica Bay and may help explain the observed difference in kelp abundance between the Bay's two coastlines.